To investigate reasons why people do not work, the Census Bureau interviewed a group of randomly selected individuals, from April-July 1996, in four separate rotation groups. Respondents were asked to select 1 of 11 categories consisting of economic and noneconomic reasons for not working, in response to the question, "What is the main reason you did not work at a job or business [in the last four months]?"
a Identify the type of study described here.
b Identify the population of interest.